1. Field of the Invention
This invention relates to a fluorescent material and an illumination device composed of the fluorescent material.
2. Description of the Related Art
Nichia Corporation started to produce white light emitting diodes (white LEDs) since 1996. U.S. Pat. No. 5,998,925 discloses a light emitting device that generates white light and that comprises a blue light emitting diode (blue LED) having a wavelength ranging from 450 nm to 470 nm as an illuminating unit, and a Cerium-doped fluorescent material (Y3Al5O12:Ce3+, also known as YAG:Ce3+), wherein Cerium is used as an activator. Part of blue light emitted from the illuminating unit is absorbed by YAG:Ce3+ and converted into yellowish light with a relatively broader emission spectrum (the peak wavelength is around 580 nm). Because massive yellow light irradiated by YAG:Ce3+ is capable of stimulating both red light and green light photoreceptors in human eyes, and the rest of blue light emitted from the blue light LED stimulates blue light photoreceptors, white light could be seen by human eyes.
However, such combination of blue LED and YAG fluorescent material has several defects including low color rendering index (Ra) owing to lack of red light, lower light emitting efficiency with increasing operating temperature, and poor thermal stability of illuminated light induced by high power light sources.
In order to improve the aforesaid defects, many researchers have focused on adding silicon (Si) or sulfur (S) elements into the composition of YAG. Silicon compound fluorescent materials, in which Al3+ is replaced by Si4+, have drawn more attention due to higher thermal and chemical stabilities, stronger absorption in UV zone, high purity and low cost, and abundant supply with respect to silicon or silicate materials.
US Patent Application Publication No. 2010/0142182 discloses an illumination system, comprising a light emitting device which includes a first phosphor layer, and a second phosphor layer which is separated from the light emitting device. The first phosphor layer includes a fluorescent material which contains silicon and nitrogen elements and which is represented by the following formula:(Y1-α-βLuαGdβ)3(Al5-u-vGauSiv)O12-vNv:Ce3+);wherein 0≦α<1, 0≦β<1, 0<(α+β+a+b)≦1, 0≦u≦1, 0<v<1, and 0<a≦0.2.
This fluorescent material has been modified based on the YAG structure. However, drawbacks of this silicon-containing fluorescent material include low bearing temperature, low stability, and relatively high brightness which causes over stimulation and results in fatigue of human eyes after exposure over a long period of time. Also, although nitrogen element has also been added to raise the sintering temperature, the fluorescent material still only has a sintering temperature of approximately 1500° C. Besides, the bearing temperature and stability are still insufficient and the color rendering index (Ra) of the modified fluorescent material is about 80% which is still similar to that of the original YAG. Moreover, higher quantity of the fluorescent material is needed in the illumination system.
Therefore, there is a need in the art to provide a fluorescent material that has high bearing temperature, high color rendering index, good thermal stability, and more natural light emission to avoid over stimulation of human eyes.